They have a reputation for being sinister loners, creatures that swim the seas in solitary silence before pouncing on unsuspecting prey. Think of the deadly predator in Jaws and you have the perfect stereotype of a shark.
But scientists in the United States, using a novel tagging procedure, have discovered that some shark species are really finny networkers who like to spend their time mixing and chilling out together. In some cases, the sharks were found to spend up to 95 consecutive hours together. And all without Snapchat.
The shark project is the handiwork of University of Delaware researchers who have collected tens of thousands of interactions between the 300 or so sand tiger sharks, fitted with electronic tags, over the past four years. Details of their research has just been published in the Nature journal Scientific Reports and could provide invaluable information for preserving sand tiger shark populations.
The sand tiger shark, which is related to the great white shark, is fished for its meat, liver oil and fins and is also hunted for sport and caught in beach nets and trawls. In addition, it is considered vulnerable because of its low reproduction rates, producing only one to two pups every two or three years.
The University of Delaware project was launched in 2012 with only a modest 20 sand tiger sharks involved at first. Each had an implanted acoustic transceiver fitted to study their movements. “We knew that during the summer the sand tiger population formed groups in Delaware Bay, with male and female adults and juveniles all together in the same places, sometimes very close together,” Danielle Haulsee, the paper’s lead author, told the University of Delaware website UDaily. However, it was not known where the sharks went during the other six to eight months of the year – or with whom, if anyone, they consorted, she added.
The transceivers were able to answer the question by initially focusing on two male sharks whose movements were tracked throughout the year. Crucially, the devices also recorded details of other animals carrying transmitters that came near the two sharks. These included Atlantic sturgeons, white and sandbar sharks, spiny dogfish and also lemon and bull sharks.
The two sand tiger sharks did not always travel together, it was found, but they did reconnect at various times of the year and encountered more than 50% of all the other tagged sand tiger sharks on the east coast of the United States. It was also discovered that sand tigers social behaviour changed considerably at different times of the year. After chilling out in the waters of Delaware Bay in summer, the two males tracked by the researcher headed south in the company of other males. Female sand tiger sharks take a different migration route at this time, it is presumed. Then, off the coast of North Carolina, the males, females and juveniles came together for a massive shark party around December.
Sand tiger sharks are proficient hunters of skates, rays and smaller sharks. With its three rows of sharply pointed teeth, it is also one of the fiercest looking of all sharks and has been known to attack humans, although only a few dozen occasions have been recorded, and never with any fatalities. On average, a sand tiger shark reaches a full length of between 6ft and 10ft and weighs between 200lb and 350lb.
Haulsee said that the acoustic transceiver tags provided scientists with a way to explore how an individual marine population works.“If we only studied them in Delaware Bay, we would think the sand tiger shark population is always mixed, with male and female juveniles and adults together throughout the year. Our work has shown that, seasonally, they are changing the size and composition of their groups. Now that we know this, we can start to identify places where human impacts may be affecting one portion of the sand tiger life cycle disproportionately.”
